Matthew Lowry, formerly a special agent with the agency’s Washington field office, tampered with hundreds of grams of heroin seized during drug investigations in 2013 and 2014, keeping it in his car for weeks or months, and periodically using it, according to charging documents filed in federal court in Washington, D.C., on Friday.

Lowry will plead guilty to 64 charges of obstruction of justice, heroin possession and conversion of property, attorney Robert Bonsib said. Prosecutors say Lowry faces at least seven years in prison, but Bonsib said he’ll argue for less time than that.

The accusations against Lowry, 33, caused federal prosecutors to dismiss charges against at least 28 defendants in drug cases and notify 150 more that Lowry had participated in investigations targeting them, according to charging documents.